Both character have nothing, x = 0
Richter has the axe, Maria has the cat, x = 2
Richter has the cross, Maria has the dragon, x = 3
Richter has holy water, Maria has the turtle shell, x = 4
Richter has the dagger, Maria has the fire bird, x = 5
Richter has the watch, Maria has the egg, x = 6
Both characters have the cell key, x = 7
Richter has the holy book, Maria has the song book, x = 8

Set the value to immediately get the item of choice or freeze the value to always have the selected item regardless of pick-ups.

9999 Credit
F82096:153
F82097:153

I have no idea what credit does, but I found codes to max it out

Play as Maria
F8209F:1

Make sure you have the value frozen at 1 before you start the game. This will allow you to play as Maria even if you've not unlocked her yet and even on stages where she isn't supposed to be playable. Freezing the value at 0 will force you to be Richter even if you've selected Maria, though that has no practical value, I suppose.